In recent years, as the packing densities of semiconductor devices are becoming higher, wires of circuits are becoming finer and the distances between the wires are also becoming narrower. It is necessary to planarize the surface of a semiconductor wafer that is a polishing target, and polishing by a polishing apparatus is performed as one method of carrying out such planarization.
A polishing apparatus includes a polishing table for holding a polishing pad for polishing a polishing target, and a top ring for holding a polishing target and pressing the polishing target against the polishing pad. The polishing table and the top ring are each rotationally driven by a driving portion (for example, a motor). A liquid (slurry) that includes a polishing agent is caused to flow on the polishing pad, and the polishing target that is held by the top ring is pushed against the polishing pad to thereby polish the polishing target.
If the polishing of a polishing target by a polishing apparatus is insufficient, a problem arises such as the risk of a short-circuit occurring due to insulation between circuits not being achieved, while if excessive polishing is performed, problems arise such as an increase in resistance values due to a reduction in the cross-sectional area of the wiring, or the wiring itself is completely removed and the circuit itself is not formed. Therefore, it is necessary for a polishing apparatus to detect the optimal polishing endpoint.
As one polishing endpoint detection means, a method is known that detects a change in a polishing frictional force when polishing has transitioned to a substance that is made of a different material. A semiconductor wafer as a polishing target has a laminated structure made of different materials including a semiconductor material, a conductive material and an insulating material, and the coefficient of friction differs between the different material layers. Therefore, the aforementioned method detects a change in the polishing frictional force that is caused by the polishing transitioning to a different material layer. According to this method, a time at which the polishing reaches a different material layer is the endpoint of the polishing.
A polishing apparatus can also detect a polishing endpoint by detecting a change in a polishing frictional force when the polishing surface of the polishing target becomes flat from a state in which the polishing surface was not flat.
A polishing frictional force that arises when polishing a polishing target appears as the driving load of a driving portion. For example, in a case where a driving portion is an electrically-driven motor, a driving load (torque) can be measured as a current that flows to the motor. Therefore, the motor current (torque current) can be detected with a current sensor, and the endpoint of polishing can be detected based on a change in the detected motor current (Japanese Patent Laid-Open No. 2001-198813).
However, in a polishing process to be executed by a polishing apparatus, there are multiple polishing conditions that depend on a combination of factors such as the kind of polishing target, the kind of polishing pad and the kind of polishing abrasive liquid (slurry). Among multiple polishing conditions, in some cases a change (characteristic point) in a torque current is not significantly manifested even when a change arises in the driving load of a driving portion. In a case where a change in the torque current is small, there is a risk that it will not be possible to appropriately detect the endpoint of polishing due to the influence of noise that appears in the torque current or waviness that arises in the waveform of the torque current, and consequently a problem such as excessive polishing can arise.
Conventionally, measures have been taken such as removing noise from the torque current by means of a noise filter. However, in some cases noise that is caused by hardware (a motor) cannot be removed even when a noise filter is used, and there is a problem that the S/N does not improve. There is also the problem that a change in the torque current is small.
Note that, appropriately detecting the endpoint of polishing is also important with regard to dressing the polishing pad. Dressing is performed by a pad dresser which has a grinding stone such as a diamond disposed on the surface thereof being brought into contact with a polishing pad. The surface of the polishing pad is cut away or roughened by the pad dresser to improve a slurry retention property of the polishing pad prior to the start of polishing, or to restore the slurry retention property of the polishing pad during use to thus maintain the polishing capacity.
Therefore, an object of one form of the present invention is to favorably detect a change in a torque current and improve the accuracy of polishing endpoint detection even in a case where noise cannot be removed even though a noise filter is used.
Further, an object of another form of the present invention is to favorably detect a change in a torque current and improve the accuracy of polishing endpoint detection even in a case where a change in the torque current is small.